The present invention relates generally to the field of percutaneous transluminal treatment of stenosed or narrowed arteries in the human vascular system. More particularly, the invention is directed to an embolic filter for capturing particles dislodged from a stenosis during an interventional procedure performed to improve blood flow through the stenosed artery.
Arteries can become stenotic in a number of ways. Often, a stenosis or lesion forms due to an accumulation of atherosclerotic plaque on the walls of a blood vessel. Atherosclerotic plaque can take many forms, one of which is a hard calcified substance, particles of which tend to dislodge during interventional procedures and embolize or flow freely in the circulatory system. A stenosis may also form from an accumulation of thrombus material which is typically softer than atherosclerotic plaque, but can nonetheless cause restricted blood flow in the lumen of a vessel. Like atherosclerotic plaque, thrombus material also tends to dislodge during interventional procedures. As used here, the term emboli refers to free flowing particulates whether composed of plaque, thrombus, or another material. Such free flowing emboli are dangerous since they may become lodged in a small blood vessel and occlude or partially occlude the vessel.
Various approaches have been developed to treat a stenotic lesion in the vasculature. Among the most common are balloon angioplasty and atherectomy. Balloon angioplasty is directed towards relieving the constriction in the artery by radially expanding the stenosis against the artery wall, while atherectomy attempts to remove the stenosis from the artery.
In a typical balloon angioplasty procedure, a guiding catheter is percutaneously introduced into the cardiovascular system of a patient through the femoral arteries by means of a conventional Seldinger technique and advanced within a patient""s vascular system until the distal end of the guiding catheter is positioned at a point proximal to the lesion site. A guide wire and a dilatation catheter having a balloon on the distal end are introduced through the guiding catheter with the guide wire sliding within the dilatation catheter. The guide wire is first advanced out of the guiding catheter into the patient""s vasculature and is directed across the arterial lesion. The dilatation catheter is subsequently advanced over the previously advanced guide wire until the dilatation balloon is properly positioned across the lesion. Once in position, the expandable balloon is inflated to a predetermined size with a radiopaque liquid at relatively high pressures to radially compress the atherosclerotic plaque of the lesion against the inside of the artery wall and thereby dilate the lumen of the artery. The balloon is then deflated to a small profile so that the dilatation catheter may be withdrawn from the patient""s vasculature and the blood flow resumed through the dilated artery. As should be appreciated by those skilled in the art, while the above-described procedure is typical, it is not the only method used in angioplasty.
The procedure for atherectomy is similar to that of balloon angioplasty in that a guiding catheter is introduced into the patient""s vasculature through a conventional Seldinger technique and a guide wire is typically advanced through the guiding catheter and across an arterial lesion to a point distal of the lesion. However, rather than expanding the lesion with a balloon, in atherectomy, a specialized catheter containing rotating cutting blades is used to mechanically cut or abrade the stenosis from the artery wall.
With both of the above procedures, the treated artery wall suffers a degree of trauma and in a small percentage of cases may abruptly collapse or may slowly narrow over a period of time. To prevent either of these conditions, the treatment procedure may be supplemented by implanting within the arterial lumen a prosthetic device known as a stent. A stent is a small tubular metallic structure which is fitted over a catheter balloon and expanded at the lesion site. Stents serve to hold open a weakened blood vessel and prevent the blood vessel from narrowing over time. Balloon angioplasty, atherectomy, and stenting procedures have proven successful and are widely used in the treatment of stenosis of the coronary and peripheral arteries and have for many patients rendered unnecessary invasive bypass surgery. However, all of the above procedures tend to create embolic particles which, in certain arteries, such as the carotid arteries, create a risk of ischemic stroke. For this reason, these beneficial techniques are rarely used in treating the carotid arteries, leaving invasive endarterectomy surgery as the primary treatment choice.
Embolic particles may be created during balloon angioplasty because stenoses are often formed from hard calcified plaque which tends to crack upon radial expansion. Upon cracking, emboli may be released into a patient""s bloodstream. Emboli may also be formed during a stent placement procedure as the metal struts of the stent may cut into the stenosis and shear off plaque or thrombus material. During an atherectomy procedure, for example, a constant stream of particles is cut from the stenosis which may not be totally captured by the mechanical filters or suction devices used in conjunction with this procedure. Thus, some particles are not captured and flow downstream as embolic. Certain locations in the patient""s vasculature can provide an even greater chance of creating friable plaque which can dislodge and enter the patient""s bloodstream. For example, saphenous vein grafts are of a special concern to the interventionalist. Disease in these grafts is typically a very friable plaque that can dislodge quite easily. For example, the mere act of passing the interventional devices through these vessels can dislodge embolic material which will be released into the patient""s bloodstream. When a physician performs a procedure in the saphenous vein, a filtering system which can capture the friable plaque is greatly needed.
Numerous embolic filters or traps for deployment distal of a lesion site have been proposed. The majority of these devices use some form of woven wire mesh basket to capture emboli, where the mesh is composed of square or diamond shaped cells. A typical example of the wire mesh basket type of intravascular filter is described in U.S. Pat. No. 4,873,978, entitled xe2x80x9cDevice and Method for Emboli Retrievalxe2x80x9d issued to Ginsburg. Ginsburg discloses a removable vascular filter permanently attached to a guide wire for deployment from a catheter. The filter is comprised of an expandable wire mesh basket employing diamond shaped cells. Upon deployment, the filter expands to contact the walls of the lumen, thereby obstructing the vessel and straining blood flowing through the lumen.
A variation of the wire mesh basket approach is described in U.S. Pat. No. 5,152,777, entitled xe2x80x9cDevice and Method for Providing Protection From Emboli and Preventing Occlusion of Blood Vesselsxe2x80x9d issued to Goldberg et al. This device consists of a filter having a plurality of resilient, stainless steel wire arms joined at one end so as to form a conical surface, and having rounded tips at their other ends to prevent damage to the vessel walls. Each arm is wound with wire in a form similar to a coil spring. Goldberg proposes that emboli entrained in blood flowing past the spring arms will be caught in the coils of the arms.
Prior art wire mesh filters have several drawbacks. The most significant of which is the relatively large cell size of the mesh. Embolic particles with nominal diameters larger than 150 microns pose a serious risk of occluding or partially occluding fine vasculature. A very fine wire mesh basket may have cells with openings about 3000-4000 microns on a diagonal across the square or diamond shaped cell. Thus, wire mesh filters may be unable to trap such small embolic particles and may be unsuitable for use in the treatment of lesions in the carotid arteries where emboli produced by an interventional procedure have a short flow path to the small diameter vessels of the brain. Moreover, other drawbacks of these types of filtering includes relatively high pressure drop, which increases with finer mesh sizes. These devices also have limited capacity as the fine pores can become clogged relatively quickly.
What is needed is a reliable filtering device that may be placed distal of an arterial lesion and used in conjunction with balloon angioplasty, atherectomy, stenting, or other interventional procedures. The device should be able to reliably trap any embolic debris with a nominal diameter larger than 150 microns and thereby render the above named procedures safe for treating lesions in the carotid arteries. Further, the device should be relatively easy to deploy and remove from the patient""s vasculature. The present invention meets these and other needs.
The present invention provides an improved intravascular filter for capturing embolic particles entrained in blood flowing in an arterial vessel during an interventional procedure. The filter is intended to be used as a primary filtering device in conjunction with interventional treatment procedures such as balloon angioplasty and/or stenting. The filter may also be used as a secondary filtering device in conjunction with a suction catheter in atherectomy and other stenosis removal procedures. The filter is capable of capturing embolic particles at least as small as 150 microns in diameter, thereby dramatically increasing the safety of balloon angioplasty and stenting. As a result, balloon angioplasty and stenting may be more frequently used in the carotid arteries where the risk of stroke from embolic particles is exceptionally high.
The filter of the present invention includes a strut assembly and a filtering medium. The strut assembly is compressible to an initial low profile delivery diameter and is expandable to a larger deployed diameter. The strut assembly is composed of a plurality of struts which may be made, for example, from spring steel or from a shape memory alloy. More specifically, the strut assembly includes an elongated cylindrical center portion and conical end portions which are shaped as truncated cones, terminating at proximal and distal collars. Attached to the strut assembly is the filtering medium which may be formed from either an open cell, porous, bio-compatible polymer foam, or a felted polymer fabric.
The filter element operates as a depth filter where embolic particles are trapped within the pores of the filter medium. Depth filters allow a high blood flow rate as well as fine filtration by utilizing layered filter media. In one embodiment, the filter element includes three media layers of depth filter. The top layer serves to capture large particles while presenting minimal resistence to blood flow. The second layer has a density greater than the first layer to capture smaller embolic particles. The bottom layer has even a greater density than the preceding layers to capture even smaller embolic particles. The bottom layer captures very fine embolic particles, however, due to its thinness and the fact that it receives blood substantially free of large embolic particles, the bottom layer does not significantly impede blood flow therethrough. Thus, the filter of the present invention may capture extremely small diameter embolic particles while still allowing a sufficient quantity of blood flow to prevent ischemic. Moreover, the depth filter, which relies on a xe2x80x9cwebxe2x80x9d of media that has a certain density, creates a filtration device which helps prevent the embolic particles from being released from the filter as the filter is being removed from the patient""s vasculature. This is due to the fact that the particles become entrapped in this xe2x80x9cwebxe2x80x9d of media and become somewhat entangled in the filter. In an embolic filtering device which utilizes, for example, a sheet of filtering material, rather than a web, there is a chance that the emboli can be xe2x80x9cbackwashedxe2x80x9d into the patient""s bloodstream as the filter is being collapsed for withdrawal from the body lumen. This scenario is actually detrimental to the patient since emboli which should have been captured by the filter are now released into the bloodstream.
The filter may be delivered to a desired location within an artery by means of a guide wire and a delivery sheath. The filter can be rotatably attached to the guide wire by the proximal collar of the strut assembly. The distal collar of the strut assembly slides axially over the guide wire and is also rotatable on the guide wire as well. This allows the strut assembly to move between its collapsed and expanded positions while still allowing the filter to freely rotate or xe2x80x9cspinxe2x80x9d about the guide wire. The attachment of the proximal collar of the strut assembly to the guide wire allows the restraining sheath to be retracted from the filter and permits a recovery sheath to be placed over the expanded strut assembly to move the strut assembly back to the collapsed position when the embolic protection device is to be removed from the patient""s vasculature.
Other features and advantages of the present invention will become more apparent from the following detailed description of the invention, when taken in conjunction with the accompanying exemplary drawings.